1. Field of the Invention
This invention relates to electro-optic modulators and more specifically to an electro-optic modulator capable of high speed linear operation.
2. Description of the Related Art
Optical modulators are used in fiber optic communication systems. As the speed of these systems increases, optical modulators with broader modulation bandwidths are required.
A common optical modulator is a directional coupler modulator fabricated on a lithium-niobate substrate. A detailed description of this device can be found in R. V. Schmidt, "Integrated Optics Switches and Modulators," Integrated Optics: Physics and Applications, ed. S. Martelluci and A. N. Chester (New York: Plenum Press, 1981), pp. 181-210.
In a directional coupler modulator, illustrated in FIG. 1, two parallel waveguides 10 and 12 are fabricated on a lithium-niobate substrate 14 in close proximity so that light 16 launched into one waveguide (the reference arm) couples to the other waveguide (the signal arm) via evanescent coupling. If the waveguides have the same propagation constants, light launched into the reference arm will transfer completely to the signal arm in a distance 1=.pi./2.kappa., where .kappa. is the coupling coefficient which describes the strength of the interguide coupling. Electrodes 18 and 20 are placed over the waveguides 10 and 12 in the coupler region.
As illustrated in the cross-sectional view of FIG. 2, when a voltage is applied across the electrodes, the electric field lines 22 normal to the substrate 14 surface are oppositely directed in each waveguide. The oppositely directed electric fields produce a phase mismatch (or propagation constant mismatch) by increasing the refractive index in one guide and decreasing it in the other through the linear electro-optic effect. The degree of light transfer along a given length via evanescent coupling depends on the difference in propagation constants between the waveguides. Therefore, the optical switching can be controlled with the applied electric fields.
The speed at which an electro-optic coupler can operate is limited by the transit time of the light through the waveguide. Ideally, an optical wave that is launched into the coupler will see a constant electric field strength as it travels the length of the coupler. However, at very high modulation frequencies (RF frequencies), the travelling optical wave will be subjected to a time-varying electric field as it travels through the waveguide.
A travelling-wave modulator, illustrated in FIG. 3, overcomes this problem to an extent by applying the voltage (with a voltage source 24) at one end of the active electrode 18, which serves as a transmission line for the electrical wave. The other end of the active electrode 18 is terminated in a load impedence 19 equal to the electrode's transmission line impedence to prevent the reflection of travelling electrical waves back toward the source 24. If the velocity of the travelling electrical wave matches that of the optical wave, transit time effects can be eliminated. However, the bandwidth of travelling-wave modulators is limited because of a velocity mismatch between the optical wave and the electrical wave at RF modulation frequencies. At RF frequencies, the index of refraction for RF electrical waves (typically about 3.5 to 4.0) is higher than the index of refraction for the optical wave (2.15 for an optical wavelength of 1.3 microns). As a result, the velocities are not matched and the optical wave experiences a changing electric field as it propagates through the modulator.
The velocity mismatch problem also impacts linearized directional-coupler modulators. Linearized directional-coupler modulators, such as the one illustrated in FIG. 4 and described in Juan F. Lam and Gregory L. Tangonan, "A Novel Optical Modulator System with Enhanced Linearization Properties", IEEE Photon. Tech. Lett., vol. 3, No. 12 (1991), pp. 1,102-1,104 and in Juan F. Lam and Gregory L. Tangonan, "A Novel Optical Modulator System with Enhanced Linearization Properties: An Erratum", IEEE Photon. Tech. Lett., vol. 4, No. 6 (1992), p. 670, are a special class of modulator that reduce nonlinear distortion effects that are common in standard modulators. This type of modulator has an optical directional coupler (10 and 12) approximately twice as long as the directional coupler of FIG. 1 and an extra set of electrodes 26 and 28 for applying a DC bias voltage 30 to the waveguides.
If the DC bias voltage 30 is set to zero, then the energy transfer curve is the same as for a standard travelling wave modulator, as illustrated in FIG. 5a. FIG. 5a illustrates the energy transfer curve for a standard travelling-wave modulator, such as the modulator of FIG. 3, whose waveguides are two coupling lengths long (light launched into the reference arm 10 completely couples to the signal arm 12 and back to the reference arm 10 before exiting the coupler, when no voltage is applied). This curve shows the amount of light that exits the signal arm as a function of the applied voltage. It is apparent from this graph that the amount of light that exits the signal arm varies nonlinearly with applied voltage. This presents a problem if one wants to modulate the output light at high frequencies, at which the nonlinear nature of the energy transfer curve results in second harmonic and third-order intermodulation distortion of the output signal.
Referring back to FIG. 4, a DC voltage 30 can be applied across electrodes 26 and 28 such that the energy transfer curve exhibits a linearized region, as shown in FIG. 5b. If the modulator is biased to the midpoint of the linearized region 32, high linearity modulation can be achieved. However, the high linearity modulation degrades at high modulation frequencies due to the velocity mismatch phenomena discussed above. As the linearity degrades, nonlinear distortion effects appear.
Some prior modulators, such as those described in D. W. Dolfi and T. R. Ranganath., "50 GHz Velocity-Matched Broad Wavelength LiNbO.sub.3 Modulator with Multimode Active Section", Electronics Letters, Vol. 28, No. 13 (1992), pp. 1,197-1,198 and in G. K. Gopalakrishnan et al., "40 GHz, Low Half-Wave Voltage Ti:LiNbO.sub.3 Intensity Modulator", Electronics Letters, Vol. 28, No. 9 (1992), pp. 826-827, have attempted to match the RF index of refraction to the optical index of refraction by using gold electrodes that are 10 microns or more thick. The thicker electrodes lower the effective RF index, but these modulators are of the interferometric variety which do not utilize evanescent coupling between the waveguides as the modulation mechanism. As a result, the waveguides can be separated by 10 microns or more, which facilitates the fabrication of thick gold electrodes using gold plating techniques.
In contrast, directional-coupler modulators that utilize evanescent coupling must have waveguides that are typically separated by 6 microns or less. In addition, the waveguides in directional-couplers are typically 2 to 3 cm long. It is difficult to reliably fabricate thick gold electrodes over waveguides that are that close together with uniform thickness over a 2 to 3 cm length. As a result, device yields go down when the thick electrode technique is utilized to match the indices of refraction in directional-coupler modulators.
Another technique for matching the RF and optical indices of refraction is described in Kazuto Noguchi et al., "A Ti:LiNbO.sub.3 Optical Intensity Modulator with More Than 20 GHz Bandwidth and 5.2 V Driving Voltage", IEEE Photon. Tech. Lett., Vol. 3, No. 4 (1991), pp. 333-335. The modulator used by Noguchi, a cross-section of which is illustrated in FIG. 6, is a Mach-Zehnder interferometric modulator (as opposed to a directional-coupler modulator) with symmetric ground electrodes 34 positioned on each side of an active electrode 36. One of the two waveguides 37 is positioned underneath the active electrode 36 and the other is positioned underneath one of the ground electrodes 34.
The index matching is achieved by placing a top metal shield 37 over the two waveguides 38, with the metal shield in electrical contact with both ground electrodes 34. The metal shield 38 is fabricated separately and cemented to the ground electrodes 34 with adhesive. The air gap 40 between the metal shield 38 and the active electrode 36 lowers the effective RF refractive index because of the low dielectric constant of air (1.0).
How well the RF and optical indices match is a function of the size of the air gap 40 created by the metal shield 38. The separate fabrication and cementing steps involved in attaching the metal shield to the ground electrodes creates a greater probability that the indices will not be precisely matched and also an increased probability of variation from one device to the next (reduced reproducibility), which can result in lower device yields.
In the linearized directional-coupler modulators discussed above, the linearity of the modulator is more sensitive than the modulation bandwidth to velocity mismatch. Therefore, the RF and optical indices of refraction must be as closely matched as possible to preserve the modulator's linearity at high modulation frequencies. The Noguchi modulator is not a linearized modulator and does not require as precise a velocity match as linearized directional-coupler modulators. Therefore, although the separately fabricated and cemented metal shield in the Noguchi modulator gives sufficient control over the RF index of refraction to increase the modulation bandwidth, it is not precise enough to reproducibly achieve the velocity matching required for linearized directional-coupler modulators.